I look forward to the results, no matter what they are.
If you didn't want to further hijack the other thread, why didn't you leave it at "See my comments in..." instead of hijacking it further? I can see why you seem so upset at everything I suggest. Every time I give one or more examples of what's possible, you seize one and rail against it as dogma. There's a big difference between claiming dogma, and suggesting options. I'm only suggesting options.
Your scenarios about Rolls batteries and warranty claims in foreign ports are way too creative for me. If I needed a battery in some foreign port, and wanted it via warranty, I'd look first for local agents. Rolls might have them. If they didn't, I'd ask them to ship the new cell DRY to the nearest US port via Parcel Post or similar economy means, not demand they needlessly ship them as overnight hazmat. Yes, battery acid can be obtained in most places where you find cars and telephones.
But I don't think I'd bother with all that fuss in the first place. I suspect that in any Caribbean port, you'll find there are port industries and fork lifts, and along with them, a convenient local way to buy the one cell you need to replace. You're way too zealous about seizing on one idea and ignoring the options.
Like, insisting parallel batteries must be better simply because a lot of folks were taught to use them. Convenient? Maybe. Cheap? Maybe. But that's like saying automotive alternators are the best ones to use on boats--because so many boatbuilders equip all their new vessels with them.
So let's try to make this clear: If you have a twelve volt battery on your boat, any kind of 12v battery on your boat, you ALREADY HAVE ONE SET OF CELLS IN SERIES. Anything you add to that, adds potential problems. Instead of adding the potential problems, if you simply choose the original six cells for adequate sizing and other constraints, you have the simplest and most reliable 12v battery.
Maybe not the cheapest, certainly not the easiest to obtain. But simplest, most reliable, and with a little perseverance about looking for the options, quite possible the best range of size, fit, weight, installation configurations, and other features.
You can't make one 12V battery, unless you make one SERIES CELL BANK. If you reject the validity of series cells...you're stuck at 2.2v for your ship's power standard. (Or 1.5, or 4.3, etc. whatever chemistry you choose.)